The invention relates to an exhaust gas turbocharger for an internal combustion engine.
Such an exhaust gas turbocharger is described in the publication DE 197 52 534 C1. The exhaust gas turbocharger comprises a compressor, which is arranged in the induction system of the internal combustion engine and is connected by means of a shaft to an exhaust gas turbine located in the exhaust system of the internal combustion engine, which exhaust gas turbine is driven by the exhaust gases, of the internal combustion engine, which are at an increased exhaust gas back pressure. The compressor then induces ambient air and compresses the latter to an increased boost pressure, at which the combustion air is supplied to the internal combustion engine.
The exhaust gas turbine is equipped with a variable turbine geometry which permits the inlet flow cross section in the turbine to be adjustably set relative to the turbine rotor. Both during engine braking performance and in the fired propulsion mode of operation, this opens the possibility of covering a relatively wide performance spectrum for generating engine braking power and propulsion power. In this arrangement, the variable turbine geometry is converted into a back-pressure position, which reduces the inlet flow cross section, during engine braking operation. By this means, the exhaust gas back pressure is increased and the pistons of the internal combustion engine have to perform additional compression work against the exhaust gas back pressure. In the fired propulsion mode of operation, the variable turbine geometry is usually converted into an open position, which extends the inlet flow cross section and in which the maximum exhaust gas flow through the exhaust gas turbine is made possible.
The variable turbine geometry is embodied as a guide cascade, which is arranged in the inlet flow cross section and to which guide vanes are pivotably fastened, the guide vanes extending over the periphery of the guide cascade and each having a pivoting axis parallel to the axis of rotation of the supercharger. In the back-pressure position, the guide vanes are pivoted into a position in which only a minimum flow cross section is freed between two adjacent guide vanes. In the open position, on the other hand, there is a maximum flow path between adjacent guide vanes.
Because of the high exhaust gas back pressure and the reduced flow cross section between adjacent guide vanes during engine braking operation, the problem can arise that compression shocks occur in the supersonic range in such guide cascades; these impinge on the turbine rotor and can lead to destruction of the turbine rotor blades. Efforts are therefore made to attenuate compression shocks in the exhaust gas flowing onto the turbine rotor but without impairing the engine braking effect, which latter presupposes a restricted inlet flow cross section. Although such compression shocks can be reduced by means of a relatively large distance between guide cascade and turbine rotor, this leads to a large design, which is not usually realizable because of the restricted space relationships in the engine compartment or in the internal combustion engine.
From the dissertation “Strömungssimulation zur optimierten Gestaltung von Turbomaschinenkomponenten” (Flow simulation of the optimized design of turbomachine components) by A. W. Reichert, Duisburg, 1994, it is known art—in the case of large-sized, stationary turbomachines—to provide a guide cascade ring which contains guide vanes and radially encompasses the turbine rotor. On their inner surface, these guide vanes have a flow edge, which effects a multiple reflection of the compression shock of the entering gaseous medium, by which means the compression shock is attenuated. This dissertation does not, however, provide for an application to small-sized exhaust gas turbochargers.